1. Field of the Disclosure
The disclosure is related to the field of electromagnetic induction or propagation well logging for determining the resistivity of earth formations penetrated by wellbores or geosteering applications. More specifically, the disclosure addresses the problem of simplifying the structure of induction or propagation logging tools.
2. Description of the Related Art
Electromagnetic induction or propagation resistivity instruments can be used to determine the electrical conductivity of earth formations surrounding a wellbore. An electromagnetic induction well logging instrument is described, for example, in U.S. Pat. No. 5,452,761 issued to Beard et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. The instrument by Beard includes a transmitter coil and a plurality of receiver coils positioned at axially spaced apart locations along the instrument housing. An alternating current is passed through the transmitter coil. Voltages which are induced in the receiver coils as a result of alternating magnetic fields induced in the earth formations are then measured. The magnitude of certain phase components of the induced receiver voltages are related to the conductivity of the media surrounding the instrument.
In many induction or propagation logging instruments, measurements are made at discrete frequencies. i.e., the transmitter operates at a selected frequency and the receiver is designed for operation at the selected frequency. In order to satisfy this criterion, tuning circuits are provided for the transmitter and the receiver. Due to the inductive nature of the antenna impedance, the tuning circuit is a capacitor with a capacitance selected to provide resonance at the selected frequency.
For an array induction or propagation logging device, this means that a tuning circuit is provided for each of the receiver antennas. Needless to say, this complicates the field calibration that is needed before the instrument can be deployed in a borehole.
The necessity of having a tuning circuit for each receiver antenna is exacerbated for logging while drilling (LWD) applications where space is at a premium and the necessity of carrying a number of tuning circuits in a so-called “J-Box” becomes burdensome.
The problem also exists in multi-component logging tools. See, for example, U.S. Pat. No. 6,553,314 to Kriegshauser et al., and U.S. Pat. No. 6,885,947 to Xiao et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. In such logging tools, the number of receivers is fewer than in a multi-array logging tool, but nevertheless, three components of data are typically measured. FIG. 4 shows an exemplary configuration of transmitters and receivers in a multicomponent tool.
Another complicating factor is the increasing use of multi-frequency measurements. See, for example, U.S. Pat. No. 7,031,839 to Tabarovsky et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. Multifrequency measurements have several uses including correction for invasion of the formation and, in LWD tools, compensating for tool conductivity and correcting for eccentering of the tool within the borehole. See, for example, U.S. Publication No. 20080033654 of Bespalov et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. It goes without saying that multifrequency operations require more tuning circuits than single frequency operation.
The present disclosure is directed towards avoiding most, if not all, of the complexities resulting from the use of tuned receivers as discussed above.